There has been conventionally known a method of cleaning substrates such as semiconductor wafers (also referred to as “wafer” below) with the use of a mixture liquid of a sulfuric acid and a hydrogen peroxide solution as a cleaning liquid (see, JP5-166780A, for example). To be specific, a wafer is cleaned by fully decomposing a resist adhered to the wafer, by means of a peroxomonosulfuric acid (H2SO5) which has been generated by mixing a sulfuric acid and a hydrogen peroxide solution (this theory will be described below).
With reference to FIG. 6, a chemical-liquid mixing apparatus for producing a mixture liquid of a sulfuric acid and a hydrogen peroxide solution is described. FIG. 6 is a schematic structural view showing a structure of a general chemical-liquid mixing apparatus.
As shown in FIG. 6, the general chemical-liquid mixing apparatus includes an inner tank 10 in which a wafer is accommodated to be cleaned, and an outer tank 12 disposed around a circumference of the inner tank 10, into which a liquid overflowing from the inner tank 10 flows. There is disposed a return pipe 14 for returning a liquid in the outer tank 12 into the inner tank 10. In the return pipe 14, there are serially arranged a return pump 16 for sending a liquid in the outer tank 12 to the inner tank 10, a dumper 18 for reducing vibrations or the like of the return pipe 14, a heater 20 for heating a liquid passing through the return pipe 14, and a filter 22 for filtering a liquid passing through the return pipe 14. The chemical-liquid mixing apparatus also includes a sulfuric-acid storage tank 24 for storing a sulfuric acid (H2SO4), and a hydrogen-peroxide-solution storage tank 30 for storing a hydrogen peroxide (H2O2) solution. The sulfuric acid and the hydrogen peroxide solution stored in the respective storage tanks 24 and 30 are supplied into the inner tank 10 by a sulfuric-acid supply pipe 28 and a hydrogen-peroxide-solution supply pipe 34, respectively. Supply operations of the respective chemical liquids can be adjusted by a sulfuric-acid supply valve 26 and a hydrogen-peroxide-solution supply valve 32, respectively.
Next, a method of producing a mixture liquid by such a chemical-liquid mixing apparatus will be described below. The inner tank 10 and the outer tank 12 are empty in their initial states.
At first, the sulfuric-acid supply valve 26 and the hydrogen-peroxide-solution supply valve 32 are opened, while the return pump 16 and the heater 20 are left OFF, so as to simultaneously supply a sulfuric acid and a hydrogen peroxide solution from the sulfuric-acid storage tank 24 and the hydrogen-peroxide-solution storage tank 30 into the inner tank 10. For example, a supply ratio between the sulfuric acid and the hydrogen peroxide solution is 5:1. Namely, a supply rate of the sulfuric acid is, e.g., 25 liters/min, and a supply rate of the hydrogen peroxide solution is, e.g., 5 liters/min. The reason why the supply rate of the sulfuric acid is considerably larger than that of the hydrogen peroxide solution is described below. The supply of the sulfuric acid and the hydrogen peroxide solution is continued until the inner tank 10 becomes full and the liquid overflows to the outer tank 12.
By supplying the sulfuric acid and the hydrogen peroxide solution into the inner tank 10, the sulfuric acid and the hydrogen peroxide solution are mixed with each other.
Mixture of the sulfuric acid and the hydrogen peroxide solution is classified into the following two patterns.
The first pattern is the following chemical reaction.H2SO4+H2O2→H2SO4+H2O+O*  Formula (1)
The reaction represented by Formula (1) generates an active oxygen (O*). This active oxygen is a strong oxidizer.
The second pattern is the following chemical reaction.H2SO4+H2O2→H2SO5+H2O  Formula (2)
The reaction represented by Formula (2) generates a peroxomonosulfuric acid (H2SO5). Similar to the active oxygen, the peroxomonosulfuric acid is also a strong oxidizer. However, the peroxornonosulfuric acid is more effective than the active oxygen in decomposing an organic matter such as a resist adhering to a wafer. That is to say, by mixing a sulfuric acid and a hydrogen peroxide solution to generate a peroxomonosulfuric acid, it is possible to fully remove a resist adhering to a wafer therefrom.
FIG. 7 is a graph in which a horizontal axis shows a ratio (molar ratio) of a sulfuric acid relative to a hydrogen peroxide solution, and a vertical axis shows a generation ratio of peroxomonosulfuric acid. As shown in FIG. 7, when the ratio (molar ratio) of a sulfuric acid relative to a hydrogen peroxide solution is raised, the generation ratio of peroxomonosulfuric acid is correspondingly increased. Thus, a resist adhering to a wafer can be more sufficiently removed. For this reason, a ratio between supply rates of a sulfuric acid and a hydrogen peroxide solution to be supplied into the inner tank 10 is set at, for example, 5:1.
After the supply of the sulfuric add and the hydrogen peroxide solution into the inner tank 10 is finished, the return pump 16 is activated to return the liquid in the outer tank 12 into the inner tank 10 through the return pipe 14. Again, the liquid overflows from the inner tank 10 to the outer tank 12. In this manner, the liquid is circulated through the combination unit of the inner tank 10 and the outer tank 12. At the same time, the heater 20 is activated to heat the liquid passing through the return pipe 14. Thus, a temperature of the liquid contained in the inner tank 10 is heated to a temperature suitable for cleaning a wafer (e.g., 100° C. to 150° C.).
After the return pipe 16 and the heater 20 are kept in ON state until a temperature of the liquid in the inner tank 10 reaches a certain temperature, the return pipe 16 and the heater 20 are again switched off. Thereafter, a plurality of wafers are all together immersed into the inner tank 10. Thus, a resist adhering to each wafer is decomposed by the mixture liquid of the sulfuric acid and the hydrogen peroxide solution, more specifically, a peroxomonosulfuric acid generated by mixing the sulfuric acid and the hydrogen peroxide solution, to thereby remove the resist from the wafer. In this manner, a series of steps for cleaning the wafer is completed.